I think maybe Androids could be programed to Act and respond with Emotional Gestures...
To make them fit in more... like with wearing a Helmet... and how David said by not wearing it would defeat the Purpose.. Holloway says they are making you very close...
David responds not to close (this is a David Emotion)
But i think Synthetics would be programed to smile, to laugh so as to fit in.
If we created Real Life Androids, i am sure it would be bland to Have Data from Star Trek TNG.
If you had a companion or servant Synthetic you would want some Facial Emotional Responses, so that it would be more pleasing.... having one that would smile at you, complement you and laugh at jokes...
I am sure when we can advance to the level to make Synthetics Real...

A Large Market would be companion's for the Rich and Famous...
And i am sure a would be owner for this reason would want to see some Emotion and Complements/Flirting scenes and interaction from them...
Rather than a basically Mechanical Mannequin
So i think Synthetics are programed to show Emotional Signs on their faces... at times.
But i think the reason for David doing so as a deliberate jibe, is something of a more Human Trait than maybe those Synthetics was intended to have.